1. Field of the Invention
The present invention relates to a pouch type lithium secondary battery which has unit cells packed in a metal outer casing in the form of a battery module.
2. Discussion of the Related Art
Generally, an electrode assembly packing material of a pouch type lithium secondary battery is formed in a multilayer film including an inner resin layer having thermal adhesion and functioning as a seal, a metal thin film layer functioning as a barrier, and an outer resin layer functioning to protect the battery from the outside. The metal thin film layer is used to prevent air, moisture, and the like from entering the interior of the battery and prevent gas generated inside the battery from leaking out, and is usually made of aluminum Al.
However, a pouch packing material of this structure may undergo external exposure of the metal thin film layer or electrical connection between the exposed metal thin film layer and an electrode lead (electrode terminal) or the like during the manufacture of a battery using the pouch packing material or during the use of the battery. For example, during the manufacture of a battery, if an excessive deformation force is locally applied or excessive thermal fusion is performed in a groove formation process for receiving an electrode assembly and an electrolyte in the pouch packing material or in a thermal fusion process for forming a sealing portion on the outer peripheral surface of the packing material, the outer resin layer or the inner resin layer may be damaged to thus expose the metal thin film layer, and the exposed metal thin film layer may be brought into electrical conduction with the battery or a metal material for another pack unit outside the battery. That is, the insulating property may be deteriorated.
Moreover, exposure of the metal thin film layer within the battery brings about a side reaction, such as peeling off the adhesive surface by causing a metal surface to be corroded by hydrofluoric acid generated by hydrolysis of lithium salt with moisture in the electrolyte.
Besides, in the groove formation (deep drawing) for receiving the electrode assembly and the electrolyte, the metal thin film layer of the pouch has low formability due to its relatively low ductility compared to the resin layer. Hence, there is a difficulty in receiving an electrode assembly of more than a predetermined thickness.
To solve these problems, there have been efforts to exclude metal layers from the pouch packing material.
For instance, Japanese Unexamined Patent Publication No. 2007-265989 discloses a battery-element package composed of a laminate film having a lamination structure of two or more resin film layers, the lamination structure having no metal foil and at least one of the resin film layers comprising a moisture absorbent material.
However, such a battery-element package is suitable for a small-sized battery. In order for the battery-element package to be used for medium and large sized batteries, each battery element needs to be packed in a package comprising a moisture absorbent material. Thus, high manufacturing costs are incurred to manufacture the battery-element package, as in the case of a conventional, high-cost metal thin film layer, and it is difficult to ensure that the penetration of gas generated internally by an abnormal chemical reaction of the battery element is prevented only by the moisture absorbent material.
On the other hand, Korean Unexamined Patent Publication No. 10-2009-0105496 discloses a laminate sheet comprising a high-strength polymer resin-based outer coating layer having excellent tensile strength, a polymer resin-based functional barrier layer capable of preventing moisture and gas penetration, and a thermally fusible polymer resin-based inner sealant layer.
However, such a polymer resin functional barrier layer is less resistant to moisture penetration compared to the conventional metal layer, requires high manufacturing costs, and has a multilayer film configuration which adds inconvenience to the process.
Consequently, the aforementioned conventional techniques exhibit very low moisture penetration resistance and gas barrier properties, and therefore have not been substantially commercialized in the industry.
Moreover, when manufacturing a pouch type lithium secondary battery module, electrode assemblies are contained in a pouch exterior material, then the pouch exterior material is thermally fused, and respective unit cells are packed further in a reinforcing material such as aluminum to obtain external strength and help heat diffusion. This poses a disadvantage in that the overall weight of the battery module increases due to the metal layer included in the pouch exterior material and the additional metal reinforcing material.
Accordingly, there has been a demand for the development of a new technique for a pouch type lithium secondary battery, which is applicable to medium and large-sized batteries, satisfies general properties, such as moisture and gas barrier properties, insulating property, formability, and prevention of peel-off due to corrosion, and is easily manufactured at low cost.